In recent years, permanent magnets have keenly been studied and developed to provide permanent magnets having high magnetic energy product for promoting compactness and high output of rotating electrical machines. In particular, rotating electrical machines for vehicles such as hybrid cars are strongly required to suppress exhaust gas, reduce fuel consumption, and improve efficiency. They are also required to reduce installation spaces, fit in limited spaces, and provide high torque and high output. To achieve them, rotating electrical machines of high energy density that need large current and magnetomotive force are expected. Realizing such rotating electrical machines, however, involves various problems.
For example, a rotating electrical machine with a rotor in which permanent magnets of high magnetic energy product are embedded produces large electromagnetic exciting force to cause vibration and noise. A rotating electrical machine for a hybrid vehicle is particularly required to realize quietness in a cabin and reduce noise to the outside. These issues must be solved.
Japanese Unexamined Patent Application Publication No. 2005-51897 (Patent Document 1) describes a rotor for a reluctance-type rotating electrical machine that achieves an effect similar to a skew effect, to reduce torque ripples, vibration, and noise. This rotor for a reluctance-type rotating electrical machine includes a rotor core formed by laminating many annular core materials whose outer circumferential parts are provided with alternating magnetic irregularities, an inner circumferential part of the rotor core being provided with an axially extending key, and a rotary shaft inserted into the inner circumferential part of the rotor core and having a key groove on an outer circumferential part thereof. The rotor core is divided into a plurality of blocks. At least among three of the blocks, the core materials that form one block are structured such that the magnetic irregularities are shifted by a predetermined angle toward one of a rotation direction and counter rotation direction with respect to a center line passing through the key. On each end of this block, the core materials of each adjacent block are formed such that the magnetic irregularities are shifted by a predetermined angle toward the other of the rotation direction and counter rotation direction with respect to the center line passing through the key.
FIG. 15 is a diametrical sectional view of the rotor described in the Patent Document 1. In an outer circumferential part of a block 3, a pair of magnet insertion holes 5 each substantially having a rectangular shape is formed such that a distance between the holes 5 gradually extends toward an outer circumference. Permanent magnets 6 are inserted into the magnet insertion holes 5 and are fixed thereto with an adhesive or a filler. In the vicinity of the outer circumference of the block 3, a hollow 7 is formed between the pair of permanent magnets 6. The hollow 7 is substantially triangular with two sides being in parallel with the pair of permanent magnets 6 and one side extending along the outer circumference.
In the block 3, a section including a pair of the magnet insertion holes 5, a pair of the permanent magnets 6, and the hollow 7 is a magnetic recess (q-axis, inter-pole section) 8 where magnetic flux hardly passes and a section between adjacent magnetic recesses 8 is a magnetic projection (d-axis, pole section) 9 where magnetic flux easily passes. The magnetic recesses 8 and magnetic projections 9 are alternately formed at intervals of predetermined angles. On an inner circumference of the block 3, two axially extending keys 30 and 31 are formed at intervals of 180 degrees.
In the block 3, a center line Lo passing through the centers of the keys 30 and 31 passes thorough the magnetic projections 9. The center line Lo deviates by an angle of Δθ from a center line Loa passing through the centers of the magnetic projections 9. Namely, the center line Loa is present at a position shifted from the center line Lo by the angle Δθ in a direction (clockwise direction) opposite to a rotation direction X. The center line Loa and a center line Lb that is adjacent to the center line Loa and passes through the centers of the magnetic recesses 8 form a predetermined angle of θ.
FIG. 16 is a partial top view of the rotor described in the Patent Document 1. The rotor has the rotor core formed by laminating many annular core materials such as silicon steel plates. The rotor core is divided into four blocks 3 and 4 having an equal thickness, the four blocks being stacked together. As illustrated in FIG. 16, the blocks 3 and 4 have center lines Lb and Lc having linear loci that are zigzagged without forming a prior-art-like straight line. As a result, the rotor provides an effect similar to a skew effect provided by a rotor for a squirrel-cage induction motor. Deviations of the center lines Lb and Lc must be ±0 (equally shifted in “+” and “−” directions).
The rotor is arranged in a stator which is not illustrated and around which a stator coil is wound, to thereby form the rotating electrical machine. In the rotating electrical machine, the rotor has the magnetic recess (q-axis) that hardly passes magnetic flux and the magnetic projection (d-axis) that easily passes magnetic flux. Spaces above the magnetic recess and projection accumulate different amounts of magnetic energy when a current is passed through the stator coil and the magnetic energy variation generates reluctance torque. The rotor also has the permanent magnets 6. Magnetic attraction and repulsion between the permanent magnets 6 and the magnetic poles of the stator generate torque. As a result, the rotor rotates in the stator.
At this time, magnetic flux at ends of each permanent magnet embedded in the rotor similarly acts with respect to the stator coil. Namely, with respect to a current passing through the stator coil, the magnetic flux at ends of each permanent magnet acts to cancel, in terms of direction and amount, an action of leakage flux of the permanent magnet, thereby suppressing axial vibration.
In this way, according to the rotor for a reluctance-type rotating electrical machine described in the Patent Document 1, a locus of a center line passing through magnetic recesses in at least one block is shifted from that of each adjacent block, to provide an effect similar to the skew effect of a rotor for a squirrel-cage induction motor and reduce torque ripples, vibration, and noise.
However, the rotating electrical machine that is compact and provides high output and high energy density has other problems in addition to the above-mentioned vibration and noise. The rotating electrical machine needs large current and magnetomotive force to provide high torque and high output. The large current passing through the armature coil applies an armature reactive magnetic field to the permanent magnets, thereby causing a problem of demagnetization of the permanent magnets.
FIG. 17 is an external view illustrating a rotor for a permanent-magnet rotating electrical machine according to the related art. The rotor 1 has a rotor core 2 and permanent magnets 6. The rotor core 2 has easy axes of magnetization and hard axes of magnetization. To create magnetic irregularities, the rotor core 2 is formed by laminating electromagnetic steel plates having permanent magnet embedding holes in which the permanent magnets 6 are embedded along the easy axes of magnetization. The permanent magnets 6 arranged in the permanent magnet embedding holes are magnetized to cancel magnetic flux of armature current passing through adjacent inter-pole sections. Namely, the two permanent magnets 6 on each side of a pole section have the same magnetization direction and the two permanent magnets 6 on each side of an inter-pole section have opposite magnetic directions with respect to a circumferential direction of the rotor 1.
The rotor core 2 described in the Patent Document 1 and illustrated in FIG. 17 is divided into a plurality (two in FIG. 17) of blocks that are stacked together. The rotor core 2 divided into the blocks has magnetic poles 17 that are shifted from each other, and with the bound stepped skew, reduces torque ripples, vibration, and noise. As illustrated in FIG. 17, a divisional skew plane 16 indicates a divisional plane of the rotor core 2 consisting of the stacked blocks.
FIG. 18 is an axially sectioned perspective view of the rotor for a permanent-magnet rotating electrical machine according to the related art. As illustrated in FIG. 18, end faces of the permanent magnet 6 are in contact with the rotor core 2. Accordingly, a demagnetizing field from the rotor core 2 due to an armature reaction acts on the end faces and corners of the permanent magnet 6. These locations are vulnerable to demagnetization, and therefore, cause demagnetization of the permanent magnet 6. FIG. 19 is a magnetic flux density distribution of the permanent magnet 6 of the permanent-magnet rotating electrical machine according to the related art. As illustrated in FIG. 19, the permanent magnet 6 has a demagnetization location 18 on an end face thereof.
A rotating electrical machine without skew also has problems mentioned below. FIG. 20 is an axial sectional view of a rotor for a permanent-magnet rotating electrical machine according to a related art. A rotor core 2 without skew is axially divided to avoid eddy current and secure strength. As illustrated in FIG. 20, a permanent magnet 6 is axially divided. If the permanent magnet 6 is magnetized in advance (premagnetized magnet), the permanent magnet 6 produces a gap 15 because it axially repulses at the divisional plane when assembled.
FIG. 21 is an enlarged axial section illustrating the rotor for a permanent-magnet rotating electrical machine according to the related art. If the rotating electrical machine is operated with the gap 15 produced between the two permanent magnets 6, the rotor core 2 applies demagnetizing field flux 26 due to an armature reaction to an end face (demagnetization occurring location 27) of the permanent magnet 6 that is vulnerable to demagnetization. The end face (demagnetization occurring location 27) of the permanent magnet 6 facing the gap 15 easily causes demagnetization.
FIG. 22 is a view illustrating a permanent magnet demagnetization state of the permanent-magnet rotating electrical machine according to the related art. Demagnetization locations 28 are locations on the permanent magnet 6 that are vulnerable to demagnetization.
To prevent demagnetization, there is an idea of thickening the magnets. The material cost of the permanent magnets occupies 30% to 40% of the cost of the rotating electrical machine, and therefore, the quantity of the permanent magnets must be minimized in terms of cost.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 2005-51897